1. FIELD OF THE INVENTION
This invention relates to improved structural elements, and more particularly relates to shell or sandwich-type composite structural elements and to improved core members suitable for use in such elements.
2. DESCRIPTION OF THE PRIOR ART
Composite structural elements of the sandwich variety, of which corrugated cardboard is a familiar example, have previously been proposed where high strength to weight ratio is desired in the formed element. In the main, core elements employed in the past can be described as derived from two-dimensional geometrical configurations swept along a third axis to yield a core element having constant sectional characteristics. Such elements, while useful for some purposes, suffer the disadvantage of susceptibility to shear or bending failure along the axis of constancy and to buckling under transverse loading normal to that axis. In partial avoidance of these difficulties resort has been had to so-called "waffle-type" cores formed from thin metal sheets embossed or dimpled into a waffle configuration of rows of square or triangular lands on either side. For example, in French Pat. No. 990,018 to Koenigs a core element having corrugations in perpendicularly opposed directions is proposed. The alternating pyramidal elevations and depressions of the Koenigs core enhance the resistance of the element to failure along selected axes but ribs bounding the pyramidal lands provide preformed paths to collapse by reason of their stress collection characteristics. Waffle-type core elements corrugated in more than two directions have been disclosed, e.g., U.S. Pat. No. 3,199,963 to Bengtsson, but such elements are subject to similar difficulties.
Commonly in shell structures it is desired that the boundary or facing sheets which enclose the core be tapering or curvilinear, e.g., in airfoils and ship hulls. As is pointed out in Structural Sandwich Composites, U.S. Government Printing Office, Division of Public Documents, D7.6/2:23 A, MIL-HDBK-23A (30 December 1968): "The waffle-type core does not lend itself well to sandwich constructions that require tapered core thickness". The repetitive sectional characteristics common to such structures suits the basic fabrication means by embossing, stamping, roll welding, etc. but require that the core surface be machined to conform to the desired curvilinear or tapered boundary sheet. In Gewiss Canadian Pat. No. 652,670, chevron core elements are prepared by folding in such fashion as to admit of later deformation or expansion in plural directions. While the result of such deformation or expansion is to provide a core element which can conform to tapered boundary sheets, the opening out of the folded configuration diminishes the included angle between core truss members and the boundary sheets and hence reduces truss support.
U.S. Pat. No. 2,738,297 to Pfistershammer discloses a core element formed of alternating hemispherical elevations and depressions protuberant from flat lands in the mid-plane of the element. This configuration has obvious advantages relative to conventionally corrugated core elements but suffers the disadvantage of discontinuous curvature through the basic inflection areas at the mid-plane. The discontinuous flat surface at that median plane must carry the membrane load from the curved surfaces of the elevations and depressions. Consequently, the moment and shear transitions impose rings of high stress coincident with the bounding of the flat lands. These bending discontinuities are sources for early buckling failures in transverse, shear or bending loads.
Accordingly, while the art of shell structure configuration has markedly progressed from mere accordian or sinusoidal corrugation in a single direction, there yet remains need for the provision of improved core elements suitable for use in shell structures, free of the stress raising discontinuities common to prior art core elements, and adapted to employment with curvilinear or tapered boundary sheets without necessitating machining or strength reduction to the end of conforming the core to non-parallel boundary configurations.